Unlikely the other element for high quality chest compression during Cardiopulmonary resuscitation(i.e. compression rate, chest recoil, hand position), chest compression depth (CCD) is influenced by surface on which the patient is placed, especially in hospital. For solving this problem, to place the patient on rigid surface, use a backboard that might decrease the mattress compression, receive a feedback that reflects the mattress compression depth (MCD) using dual accelerometer or magnetic sensor have been proposed. As the other solution, we hypothesized that training of 6-7cm CCD for healthcare providers improve accurate CCD during cardiopulmonary resuscitation when manikin is placed on mattress in hospital.